Babies who are exposed to lots of traffic-related air pollution in the womb and during their first year of life are more likely to become autistic, suggests a new study.

The findings support previous research linking how close children live to freeways with their risk of autism, according to the study's lead author.

"We're not saying traffic pollution causes autism, but it may be a risk factor for it," said Heather Volk, an assistant professor at the Keck School of Medicine of the University of Southern California in Los Angeles.

Autism is a spectrum of disorders ranging from a profound inability to communicate and mental retardation to milder symptoms seen in Asperger's syndrome.

The prevalence of autism has grown over the past few years. It's now estimated that the disorder affects one in every 88 children born in the United States, which is a 25 per cent increase from a 2006 estimate.

The increase in autism diagnoses has also been accompanied by a growing body of research on the disorder.

Including Volk's new study, there are three articles on autism in the latest issue of the Archives of General Psychiatry.

"A decade ago, the journal published about the same number of autism articles per year," wrote Geraldine Dawson of the University of North Carolina at Chapel Hill, in an editorial accompanying the studies.

The two other reports in the current issue deal with ways to image a person's brain to look for physical differences between an autistic and non-autistic brain.

According to Dawson, who is also chief science officer of the advocacy group Autism Speaks, the number of studies on autism began to grow around 2000. Most studies, she says, deal with the biology of the disease.

ENVIRONMENTAL FACTORS

Volk's new study, however, is one of a series of looks into how environmental factors may be linked to a child's risk of being autistic, done over the past few years."I think it's definitely an area that's been understudied until recently," Volk told Reuters Health.

Unlike their last study, which used how close a child lived to a freeway as a substitute for pollution exposure, for the new analysis Volk and her colleagues looked at measures of air quality around kids' homes.

Compared to 245 Californian children who were not autistic, the researchers found that 279 autistic children were almost twice as likely to have been exposed to the highest levels of pollution while in the womb, and about three times as likely to have been exposed to that level during their first year of life.

They found that children exposed to the highest amount of "particulate matter" - a mixture of acids, metals, soil and dust - had about a two-fold increase in autism risk. That type of regional pollution is tracked by the Environmental Protection Agency.

Volk and her colleagues also saw a similar link between autism and nitrogen dioxide, which is in car, truck and other vehicle emissions.

"This is a risk factor that we can modify and potentially reduce the risk for autism," wrote Dawson in an email to Reuters Health.

The researchers said certain pollutants could play a role in brain development - but that doesn't prove being exposed to air pollution makes kids autistic. They warned that there may be other factors that explain the association, including indoor pollution and second-hand smoke exposure.

"There are some potential pathways that we're examining in our current research that will be coming up next," said Volk.